Existing braking systems for railway vehicles generally comprise electro-pneumatic assemblies controlled by electronic units of the microprocessor type. The design of these braking systems is governed by specific standards (in Europe, for example, the EN 50126 standard relating to system definition, the EN 50128 standard concerning software design and development, and the EN 50129 standard relating to hardware specifications and design). These standards introduced the concept of “Safety Integrity Level” (SIL hereafter) which defines the degree of reduction of risk to human safety that can be associated with a given function relating to a braking installation.
A braking installation for railway vehicles is designed to execute a plurality of functions, for example (but not only) service braking, parking braking, safety braking, emergency braking, braking correction in case of wheel sliding or locking (wheel slide protection), and holding braking.
A different SIL level is required for each of these functions: in particular, the emergency braking and safety braking functions must be implemented with safety levels in the range from SIL=3 to SIL=4, with reference to a scale running from a minimum of SIL=0 to a maximum of SIL=4.
In the present state of the art, purely mechanical-pneumatic solutions are used in virtually all cases to execute the emergency braking and safety braking functions, since these solutions enable the requisite SIL levels to be reached and verified in a convenient manner.
FIG. 1 of the attached drawings shows, by way of example, an electro-pneumatic braking installation for railway vehicles according to the prior art, in which the safety braking pressure is determined by a valve 1, commonly known as an LPPV (Load Proportional Pressure Valve). This valve is used to generate a braking pressure proportional to the detected weight of the railway vehicle (or of a part thereof, for example a bogie), in order to provide the greatest possible deceleration within the limits of wheel-to-rail adhesion defined at the design stage. The valve, various implementations of which are known, executes a transfer function of the type shown in qualitative terms in FIG. 2, where the pressure Pi at the input of the valve 1 is shown on the horizontal axis, and the pressure Po at the output of this valve is shown on the vertical axis. According to FIG. 2, when the pressure Pi varies between a value Ptare and a maximum value Pimax, the output pressure Po varies between a minimum value Pomin and a maximum value Pomax, along a straight line characterized by a slope angle α. Additionally, when the pressure Pi varies between Ptare and 0, the output pressure Po varies between the value Pomin and an intermediate value P*o, according to a straight line characterized by a slope angle β. The pressure P*o is such that the vehicle is always braked if a fault occurs in the suspension, such that an excessively low pressure value is caused, as shown in the broken-line continuation of the straight line having the slope α.
With reference to FIG. 1 again, the pressure Po at the output of the valve 1 is sent (for example) to the control chamber of a relay valve 2, through one or more solenoid safety valves 3. These solenoid valves 3 are normally in the state of pneumatic conduction when de-energized, and are energized by a safety loop of the braking system. Safety braking is applied by de-energizing the safety loop, the pressure Po from the output of the valve 1 then being propagated by the control chamber of the relay valve 2, which amplifies its power, at its output 2a, towards the brake cylinder or cylinders (not shown).
The known solution described above is one of various possible solutions used to execute a braking function with a safety level equal to or greater than the SIL 3 level defined in the EN 50126 standard.
Although these solutions are satisfactory in terms of the safety level, they have considerable drawbacks due to the complexity and nature of the devices and components used, such as springs, rubber diaphragms, sealing rings, and the like. The use of these components has a negative effect on the accuracy of the functional characteristics provided, and on their repeatability when the operating temperature varies, in view of functional requirements which commonly specify operating temperature ranges from −40° C. to +70° C. Additionally, the provision of operating characteristics such as those shown in FIG. 2 by purely mechanical-pneumatic means requires complicated solutions, such as specific ratios between the rubber diaphragm surfaces and the spring loading, these ratios determining the slope angle α, β and the points of intersection of the straight lines with this Cartesian axes.
Also, with the known solutions of the purely mechanical-pneumatic type, it is substantially impossible to calibrate the operating characteristics on board a vehicle during the normal adjustment of the vehicle (during commissioning), and therefore, if the slopes α, β or the pressure values at the points of intersection of the straight lines with the Cartesian axes have to be varied, the ratios between the surfaces of the rubber diaphragms and the spring loadings must be completely replanned, which will obviously create delays in the adjustment of the vehicle.
Furthermore, the variation of the aforesaid functional characteristics due to the tolerances of the materials and the fluctuations caused by temperature variations and ageing results in a considerable lack of precision in the stopping distances of railway vehicles during emergency and/or safety braking.
It is also known that the use of microprocessor systems for the feedback control of pneumatic solenoid valves enables the characteristic function of the valve 1 described above to be reproduced conveniently, while providing much greater accuracy than that allowed by existing mechanical-pneumatic components, over a range of temperature and time variations, thus making the aforesaid stopping distances much more precise and repeatable. Moreover, certain characteristics such as the slopes α and β can be easily and rapidly modified simply by using software methods to reprogram parameters.
FIG. 3 of the appended drawings shows an embodiment of an electro-pneumatic assembly 10 for controlling the pneumatic pressure in a chamber or volume 11, such as the volume of a brake cylinder, or the control chamber of a relay valve which controls the supply of pressure to the volume of a brake cylinder. This assembly 10 comprises a solenoid supply or filling valve 12 adapted to connect the chamber 11 selectively to a pressure source PS or to the atmosphere, and a vent or discharge valve 13 adapted to allow or selectively prevent the connection of the chamber 11 to the atmosphere. The solenoid valves 12 and 13 are provided with respective control solenoids 12a, 13a to which respective electronic switches are coupled in the manner described below.
The chamber or volume 11 is connected to a conduit 14 which connects the output of the solenoid valve 12 to the input of the solenoid valve 13.
When the solenoids 12a and 13a of the solenoid valves 12 and 13 are de-energized, these solenoid valves appear in the condition shown in FIG. 3: the volume or chamber 11 is connected to the atmosphere, and the pressure within it is reduced to the value of atmospheric pressure.
When the solenoid valves 12 and 13 are both energized, the first valve supplies the chamber 11 with a flow of air taken from the pressure source, while the second valve disconnects the chamber 11 from the atmosphere. Thus the pressure in the chamber 11 is increased.
When the solenoid valve 12 is de-energized and the solenoid valve 13 is energized, the chamber 11 is disconnected both from the pressure source and from the atmosphere, and the pressure within it remains substantially unchanged.
The behaviour of the electro-pneumatic assembly 10 of FIG. 3 with the variation of the conditions of energizing and de-energizing of the solenoids 12a and 13a is summarized in Table 1 below.
TABLE 112a13aPressure in 1100DECREASE01MAINTENANCE11INCREASE10—0 = de-energized1 = energized— = condition not used
By suitably modulating the energizing conditions or states of the solenoid valves 12 and 13 shown in Table 1, it is possible to produce and maintain in the volume or chamber 11 any value of pressure between the pressure PS of the source and atmospheric pressure Patm.
FIGS. 4 and 5 show variant embodiments of the electro-pneumatic assembly 10. In these figures, parts and elements identical or corresponding to those described previously have been given the same reference numerals as those used previously.
The mode of operation of the electro-pneumatic assemblies 10 of FIGS. 4 and 5 can be summarized as shown in Tables 2 and 3 below.
TABLE 212a13aPressure in 1100MAINTENANCE01DECREASE10INCREASE11—
TABLE 312a13aPressure in 1100INCREASE10MAINTENANCE11DECREASE01—
Once again, in the case of the electro-pneumatic assemblies 10 of FIGS. 4 and 5, by suitably modulating the energizing conditions or states of the solenoid valves 12 and 13 it is possible to produce and maintain in the volume or chamber 11 any value of pressure between PS and PATM.
FIG. 6 shows, in the form of a block diagram, an electronic control system 15 according to the prior art, for controlling an electro-pneumatic assembly according to one of FIGS. 3 to 5. This system 15 essentially comprises a processing and control unit 16, of the microprocessor or microcontroller type, which receives at an input a signal L containing information on the weight of the vehicle (or of a single bogie of the vehicle), for example the instantaneous value of the pressure Pi shown on the horizontal axis of FIG. 2.
At another input, the unit 16 receives a signal P representing the pneumatic pressure within the volume or chamber 11, detected by means of a suitable sensor. The unit 16 may receive further signals or input data II, which are not essential for the purposes of the present description.
By means of bias circuits 17 and 18, the unit 16 controls corresponding solid-state electronic switches 19 and 20, such as p-channel MOS transistors or simple NPN transistors, which control the energizing/de-energizing condition of the solenoids 12a and 13a respectively, in parallel with which respective recirculation diodes 21 and 22 may be connected. In the control system 15 of FIG. 6, the electronic switches 19 and 20 are connected in series with the windings 12a and 13a, between a d.c. power source Vcc and the earth GND.
The unit 16 may if necessary supply further output signals OO, relating to other processes not essential for the purposes of the present description.
By implementing suitable closed-loop control algorithms, for example PID algorithms, “fuzzy” algorithms, or algorithms of the on-off type with hysteresis (also known as “bangbang” control algorithms), the unit 16 can be designed to provide the characteristic shown in the diagram of FIG. 2, in such a way that the pressure in the container or volume 11 corresponds to the pressure Po in this diagram. For this purpose, the unit 16 receives, through an input port, the values of a set of parameters PP which characterize the control algorithm. The values of these parameters are stored in a non-volatile memory of the unit 16.
As an alternative to the implementation shown schematically in FIG. 6, the solenoids 12a and 13a may be connected to the earth GND, while the associated switches 19 and 20 may be connected to the d.c. power source. In this case, the switches 19 and 20 can be n-channel MOS transistors or PNP transistors.
In view of the EN 50126, EN 50128 and EN 50129 standards, if the function implemented by the unit 16, for example the pressure characteristic according to the diagram of FIG. 2, requires a safety level equivalent to SIL 3 or SIL 4, then, since the unit 16 is the only device contributing to the execution of this safety function, the corresponding software must also be implemented with a process having a safety level of SIL 3 or SIL 4, as specified, in particular, in the EN 50128 standard. However, this software implementation process is characterized by extremely high organizational, financial and maintenance-related costs, which frequently make its use less attractive by comparison with the more conventional mechanical-pneumatic systems, even though these suffer from all the aforementioned drawbacks.